Oxytocin is a peptide hormone which consists of nine amino acid residues. Oxytocin is known to be synthesized in nerve cells located in the paraventricular and supraoptic nuclei of hypothalamus, and released from the posterior pituitary into the blood. Known biological activities of oxytocin include uterine contraction and milk ejection activities. According to reports, intranasal administration of oxytocin increases trust in others (Kosfeld M. et al., Nature, 435: 673-676 (2005)), and decreases fears (Kirsch P. et al., J. Neurosci., 25: 11489-93 (2005)). In addition, animal and human studies revealed that oxytocin in the brain controls social behaviors including generating social recognition memory and nurturing behavior, and down-regulates stress responses including anxiety neurosis (Haruhiro Higashida, Protein, Nucleic Acid, Enzyme, 52: 1832-1839 (2007); and Neumann, I. D. Cell Metabolism, 5:231-233 (2007)). Thus, in addition to known biological activities, oxytocin is involved in higher mental functions required for the maintenance of human relationships and is considered to be effective in emotional improvement.